A recovery method of metals usually involves a process of crushing scrap containing the metals and dissolving them in an alkaline solution. However, in the recovery of tungsten, applying such a conventional process is very difficult because tungsten is a very hard and highly chemical resistant metal. Therefore, it is a common method in the art that tungsten is oxidized by a strong treatment with alkali molten salts or the like to dissolve and recover tungsten (Patent Document 1, Patent Document 2 and the like).
In the common method, a tungsten-used target and an end material are originally high purity, so that the use of the molten salt treatment or pulverization which is the general treatment as described above will lead to significant reduction of purity. Therefore, in order to provide tungsten with high purity, multistage purification, ion exchange treatment and the like will be required, thereby leading to complicated treating steps.
Electrolysis is known as the method for dissolving tungsten. To recover tungsten with high purity, ammonium nitrate that is an inorganic solution would be considered to be used for the electrolytic solution. However, it is necessary to separately add ammonia to adjust a pH of the electrolytic solution in order to carry out alkaline electrolysis.
However, ammonium nitrate itself increases a risk of explosion at high concentration. It is thus necessary to control the concentration of ammonium nitrate during electrolysis. Furthermore, since ammonia also has a concentration change due to volatilization in an electrolysis temperature range, concentration control and the like are necessary, so that running costs are required for recovery equipment, control equipment and the like. In addition, an impurity-free treating method is required for the recovery of tungsten from materials having originally high purity, such as used targets and end materials.
To solve the above problems, the present inventors previously found that tungsten with high purity could be recovered with a low cost by carrying out electrolysis using an alcohol amine-containing electrolytic solution (Patent Document 3).